Welcome to WeldingWeb.com, the ultimate Source for Welding Information & Knowledge Sharing! Here you can join over 40,000 Welding Professionals & enthusiasts from around the world discussing all things related to Welding. You are currently viewing as a guest which gives you limited access to view discussions

To gain full access to our community you must register; for a free account. As a registered member you will be able to:

Participate in over a dozen different forum sections and search/browse from nearly a million posts.

Post photos, respond to polls and access other special features
All this and much more is available to you absolutely free when you register; for an account, so join our community today today!

If you have any problems with the registration process or your account login, please contact support.

plasma cutter question

Can someone explain the different methods that Plasma Cutters start the arc. IE; Blow back starting, pilot start, hi and low frequency starting, and drag starting. Also there does not seem to be lot to wear out. How does a MOSFET semi-conductor wear out. I think the big factor on life expectancy is not respecting the duty cycle and overheating the unit. What do I know. I don't even own a plasma cutter but I want one badly. I made my own welder but a plasma cutter is more complicated.

Re: plasma cutter question

There is blow back starting as well as high frequency starting technologies (no such thing as "low frequency" start, but I have seen mention of it by some un-informed sales types!). Both of these gas ionization methods can be used to develop a "pilot arc" (a non-transferred plasma arc that can fire in the air, away from the workpiece). Some older technology torches require a scratch start....physically scratching the torch nozzle on the workpiece....which is a non-pilot arc starting method. Here is an explanation I wrote a number of years ago:

A plasma torch needs a method of heating the gas (air in an air plasma torch) up to its ionization temperature......when this occurs the gas becomes electrically conductive, which allows the DC energy from the plasma power supply to superheat the air into a plasma gas, which creates the high temperature cutting process.

There are two popular ways to ionize the gas in a torch...."High Frequency"discharge, and blowback start method.

High frequency starting uses a high voltage (around 15,000 volts) high frequency AC electrical source to create a high intensity spark inside the torch to heat the plasma gas. This discharge, similar to the power used to fire spark plugs in an internal combustion engine, is developed using circuitry in the plasma power supply, and is transmitted through wires in the torch leads to the torch. High frequency plasma systems (as well as tig welders with high frequency) have been known to cause issues with sensitive electronic devices. Many cnc machines that use a standard PC as their controller often have issues with electrical noise interference that is caused by this starting method. Industrial CNC machines often are designed with complex grounding and filtering that allows high frequency plasma systems to be used without issues.

1.Some high frequency torches are designed to fire a "pilot arc" which is a high intensity arc that essentially will fire in the air. These types of torches work well with transferring the arc to rusty or painted surfaces. Once the pilot arc makes electrical contact to the plate being cut..the main plasma power ramps up and the cutting process begins.

Many older technology air plasma systems uses this high frequency/pilot arc starting method. All high end industrial plasma systems currently use this method of starting....as it allows for better torch accuracy (cut quality) and starting reliability in high duty cycle applications.

2.Other high frequency torch designs (generally very low cost copies of old plasma technology) use high frequency to create the arc through making physical contact with the plate. These torches, often know as high frequency start or scratch start must actually have the nozzle in extremely close proximity to the plate, or on dirty, rust plate must scratch through the surface of the plate in order to allow the high frequency discharge to make contact and ionize the gas.

This technology is generally reserved to very low cost hand plasma cutting systems. It is difficult at times to get the arc to transfer to the plate, and is not generally good for mechanized cutting applications as the nozzle must be in very close proximity to the plate in order to transfer tha arc....which causes molten metal to blow back and damage the nozzle orifice.

3.The blowback start torches (developed by Hypertherm, but used by many companies today) use a moving electrode inside the torch. The main DC power is activated in the plasma torch, then moments later the gas flow starts in the torch. The gas flow causes the spring loaded electrode to slide back away from the nozzle, which creates a short circuit spark that ionizes the gas flow through the torch. These torches will fire in the air, and will penetrate through rusty, dirty, painted surfaces as well as the first method of High frequency with a pilot arc.

The majority of air plasma systems use this technique today for hand and mechanized cutting applications. This innovation allowed for smaller inverter based plasma technology systems to become easier to use, and lower priced. Since there is no high frequency discharge, these types of systems can be used on many entry level cnc machines with no need for additional grounding and filtering, and since there is a pilot arc, they can transfer to the plate from a distance...allowing for piercing on thick plate with no nozzle damage.

Jim Colt Hypertherm

Originally Posted by Splinter

Can someone explain the different methods that Plasma Cutters start the arc. IE; Blow back starting, pilot start, hi and low frequency starting, and drag starting. Also there does not seem to be lot to wear out. How does a MOSFET semi-conductor wear out. I think the big factor on life expectancy is not respecting the duty cycle and overheating the unit. What do I know. I don't even own a plasma cutter but I want one badly. I made my own welder but a plasma cutter is more complicated.

Re: plasma cutter question

Originally Posted by jimcolt

... (no such thing as "low frequency" start, but I have seen mention of it by some un-informed sales types!). ... Jim Colt Hypertherm

Jim, I take issue with that. Having been a salesman for, shall we say, "electrical goods" for many years, I've had a couple of "low frequency starts" in the morning. Too many beers after a late night in a hospitality room, you have an early presentation and all your slides are upside down in the carousel. Or catch the flu on the first day of a 4 day convention.

Re: plasma cutter question

Something that does not seem to be discussed is the role of the vortex in establishing the arc. No one would go to such extreme measures inside the tip if creating the vortex was not important. Yet... no discussion. Possibly the fluid dynamics involved is the best kept secret? I noticed that historically during the cold war research into the fluid dynamics of a vortex was declared secret.
From what I know electric arcs do not travel as well in a high pressure environment. High altitude bomber aircraft during WW2 had their ignition systems pressurized with air to prevent stray spark leakage. In order to guide a plasma stream the center of a vortex would be the easiest location for an arc to establish since it has a reduced pressure.
Comments Jim?

Re: plasma cutter question

Ha ha....yes, I seem to experience more low frequency as I age. Add some plasma (coffee) and I eventually ionize....

Originally Posted by Oldendum

Jim, I take issue with that. Having been a salesman for, shall we say, "electrical goods" for many years, I've had a couple of "low frequency starts" in the morning. Too many beers after a late night in a hospitality room, you have an early presentation and all your slides are upside down in the carousel. Or catch the flu on the first day of a 4 day convention.

Re: plasma cutter question

There is a vortex created by the gas swirl in a plasma torch, but it has little to do with getting the arc started. The gas swirl creates a vortex that can help control edge angularity, electrode life (by forcing the arc to attach dead center on the hafnium or tungsten emitter) and nozzle orifice life (centrifugally forcing cooler, un-ionized gas particles to protect the nozzle bore. I could talk for hours about this....but don't want to lose everyones interest! Many companies that produce copies of older technology torches and consumables do not understand all the high temperature physics that occur in a plasma torch....so they copy what works! Jim Colt

Originally Posted by lotechman

Something that does not seem to be discussed is the role of the vortex in establishing the arc. No one would go to such extreme measures inside the tip if creating the vortex was not important. Yet... no discussion. Possibly the fluid dynamics involved is the best kept secret? I noticed that historically during the cold war research into the fluid dynamics of a vortex was declared secret.
From what I know electric arcs do not travel as well in a high pressure environment. High altitude bomber aircraft during WW2 had their ignition systems pressurized with air to prevent stray spark leakage. In order to guide a plasma stream the center of a vortex would be the easiest location for an arc to establish since it has a reduced pressure.
Comments Jim?

Re: plasma cutter question

Splinter,

Is that AC only or AC/DC? Can you post some pictures of the beads made by that welder? I've never seen a home made welder. You can buy good used stick welders all day and Craigslist for $100 or less. How many amps can it produce? Impressive build thanks for posting.